1. Field of the Invention
The present invention relates to compact, expendable gas turbine engine, and more specifically to a gas turbine engine having a rear-mounted main bearing that is cooled by air supplied by the compressor, the air then being discharged into the combustor for mixing with the fuel.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Small gas turbine engines of the kind used in Unmanned Air Vehicles (UAV) such as a small cruise missile or drone are well known in the art. These turbines produce a thrust from less than 100 lbs to several hundred lbs. Because these turbine engines must fit within a small space, they tend to be very compact and run at high rotational speeds. One problem with prior art small gas turbine engines is the location of at least one of the main bearings that support the rotor shaft. It is preferred to locate one of the main bearings near the compressor and another of the main bearings near the turbine or combustor exit. However, the bearing near the combustor exit is exposed to very high temperatures.
Another problem with these small engines is the lack of space for storage of a bearing lubrication. These small engines must have a long storage life, and be ready to operate on a short notice. Consequently, bearings must run with little or no lubrication. Fuel lubrication, dry lubrication and grease packed bearings are typical.
To overcome this problem, the prior art small gas turbine engines locate the two bearings in the front portion of the compressor such as shown in US Patent Application Publication No. 2004/0216445 A1 entitled Combined Stage Single Shaft Turbofan Engine by Jones et al published on Nov. 4, 2004. The advantage of locating the bearings away from the hot combustor section of the turbine is that the bearings are cooler and easily lubricated, and the rotor can be a single rotor module that can be balanced as built. The disadvantages are the heavy overhung mass that results in rotor dynamics problems, slow starting, and little combustor space. Bearing lube (typically fuel) passes into the engine flow path, but no more than half can burn since it does not enter the combustor in the right place. This bearing arrangement is also limited to low bearing air flows.
U.S. Pat. No. 5,727,378 entitled Gas Turbine Engine issued to Seymour on Mar. 17, 1998 shows a small gas turbine engine in which the main bearings are located forward of the turbine and combustor, but with the rear-most bearing located between the compressor outlet and the turbine inlet, in which a portion of the compressor outlet is directed over an end plate that forms a passage for cooling air. The diverted portion of air from the compressor thus provides cooling for the rear-most bear, but the air does not pass through the bearing, and the air is discharged into the turbine downstream of the combustor. This non-contact cooling air does not provide adequate cooling for the present invention, and wastes the compressed air by discharging it into the turbine without producing any power
U.S. Pat. No. 5,526,640 issued to Brooks et al on Jun. 18, 1996 entitled Gas Turbine Engine Including A Bearing Support Tube Cantilevered From A Turbine Nozzle Wall shows a gas turbine engine with a bearing support tube and two main bearings, where both fuel and air are passed through the bearing support tube to cool and lubricate both bearings. However, the air flow through the tube is very low, and all of the fuel supplied to the combustor is delivered through the tube. The low air flow of the Brooks et al invention does not provide adequate cooling or conditioning for the bearings exposed to high temperatures from the combustor, and the large amount of fuel used for lubricating requires seals and other added structure for the bearings to prevent fuel from leaking.
U.S. Pat. No. 3,381,471 issued to Szydlowski on May 7, 1968 and entitled Combustion Chamber For Gas Turbine Engines shows a nozzle located between the combustor and the turbine, and where a portion of the compressor discharge is directed around the back end of the combustor and through the nozzle to cool the nozzle, the air flow through the nozzle then being directed into the combustor for burning. Szydlowski does not use any air flow to cool a bearing.